Detector for visual and aural direction finders



Jan. 27, 1959 C. A. GALLAGHER FIG. I.

0* RAD/0 DIRECT/0N- FINDER MEANS INTERMEDIAT! FREQUENCY FIG. 3.

//VPU7 7'0 DETECTOR 0 200 400 600 800 I090 FORWARD RES/STANCE "0"5 FIG.4.

VIDEO OUTPUT Carma/03 A. 6411.1467/51? ATTORNEY$ United States PatentDETECTOR FOR VISUAL AND AURAL DIRECTION FINDERS Cornelius A. Gallagher,Hicksville, N. Y., assignor to Servo Corporation of America, New HydePark, N. 1., a corporation of New York Application June 11, 1954, SerialNo. 435,983

8 Claims. (Cl. 343-113) My invention relates to an improved detectorcircuit having particular application to radio-direction finders.

It is an object of the invention to provide an improved circuit of thecharacter indicated.

A specific object is to provide simplified circuit means for improvingthe sharpness of nulls, used to identify direction-finder bearings.

It is a general object to meet the above objects with a structure whichinherently improves the ease of interpretability of visual and auraldirection finders, whether of the continuously rotated or manuallyrotated variety.

Other objects and various further features of novelty and invention willbe pointed out or will occur to those skilled in the art from a readingof the following specification, in conjunction with the accompanyingdrawings. In said drawings:

Fig. l is an electrical diagram illustrating a circuit embodyingfeatures of the invention;

Fig. 2 is a graphical display of a preferred characteristic of circuitelements in Fig. 1; and

Figs. 3 and 4 are wave-form diagrams illustrating operation of thecircuit of Fig. l.

Briefly stated, the invention contemplates improved means for sharplydefining the null of the received signal in a null-type directionfinder, in order to improve the hearing readability, whether thedirection finder be of the automatic visual variety or a manuallyoperated aural device. Null-sharpening is achieved through the action ofa detector circuit embodying a network of elements having non-linearproperties in the vicinity of zero volts, which is the region ofinterest in null interpretation.

Referring to the drawings, my invention is shown in application toradio-direction-finder means which may develop, in an output line 11, aradio frequency, or an intermediate frequency having envelope modulationfrom which the null is to be interpreted to identify a bearing. For thecase depicted, the signal output at 11 is the intermediate frequency,coupled by transformer means 12 to an intermediate-frequency amplifier13. The output of amplifier 13 includes a tank circuit 14, resonant atthe intermediate frequency, and coupled to display means 15 by means ofa detector circuit incorporating features of the invention.

The display means 15 may include a speaker 16 for aural presentation.Alternatively or additionally, means 15 may include a visual display 17,as presented on the face of a cathode-ray tube with the familiarpropeller pattern 18, in which the radially outer tips represent signalnulls and are read off an azimuth scale 19 to identify bearings.

In accordance with the invention, I provide a novel detector circuithaving strongly non-linear characteristics in the vicinity of zero voltsand, therefore, capable of more sharply defining the null in theenvelope of signals extracted from tank circuit 14. Without mynull-sharpening means, the envelope would appear of generally the formdisplayed in Fig. 3, in which it will be seen that nulls are not sharp.

My circuit comprises essentially a rectifier consisting of two arms20-21, which together may constitute the output or load impedance forthe tank circuit 14, and across which output connections are madethrough a resistor 22 to the display means 15; resistor 22 may functionwith the stray circuit capacity of a connection line (e. g. coaxialcable) to define a filter for smoothing the rectifier output. In orderto provide means across which a voltage can always be detected, I show aload resistor 23 in the arm 21, and have provided two non-linearrectifying elements 2425 in this arm. For best operation, I prefer thatthe combined maximum forward resistance of the elements 24-45 shall be asubstantial fraction (and preferably of the order of magnitude) of theresistance 23. The other arm 20 may include another rectifying element26 of reverse polarity with respect to the elements 24-25.

As noted above, I prefer that the maximum forward resistance of thediodes 2425 shall be of the order of magnitude of the resistor 23. Thismay be achieved by employment of one diode only, say the diode 24 inseries with a smaller resistor 23, but two diodes 2425 provide for agreater and more desirable load resistance.

in addition to the primary function of demodulating received signals,the detector network performs the secondary function of sharpening thepropeller" pattern 18; this is apparent from Fig. 4, which depicts atypical oscilloscopic observation of the output of the detector. Thesharpening at the nulls is attributable to the strongly nonlinearcharaacteristics of the rectifiers 24-2526. which may be germaniumdiodes, having characteristics as shown in Fig. 2. The non-linearcharacteristics occur predominantly in the vicinity of zero volts; theinput wave form is, therefore, substantially altered at low-voltagecurrent flow. Since this non-linear characteristic involves an increasein resistance at low-voltage levels, the current relied upon to developdetector output is markedly and non-linearly decreased for the lowervoltage levels. and sharpening results.

in operation, during the positive half of an intermediatefrcqueneycycle, the arm containing the rectifying element or diode 26 conductsand provides a low-impedance path across the tank capacitor, therebyclipping off the positive half of the cycle. On the negative half of thecycle, the other arm 2]. (containing diodes 24-25) conducts. Forrelatively high negative amplitudes of the intermediate-frequencysignal, the diodes 24-25 act as very low resistance elements. and thetotal resistance olthe arm 21 is dominated by the resistance 23. As themodulation envelope approaches the null, the resistance of arm 21 risessharply, approaching double the value of resistance 23, for smallfractions of a volt across the tank 14, thus effectively raising the (1"of the tank and enabling a larger-than-normal output directly adjacentthe null point. The filter defined by resistor 22 and the capacitance(looking into the connection line, 1:. g. coaxial cable, connectingresistor 22 to the display means 15) converts the train of negativeintermediatefrcquency pulses into the envelope shown in Fig. 4.

It will be seen that have described a relatively simple circuitproviding improved performance for null-type direction finders, Whetherof the visual or aural variety, or of the continuously rotated ormanually rotated variety. One of the outstanding features of my circuitis that overall amplitude of the input signal is relatively unimportant.

since the sharpening action takes place only in the vicinity of thenull. Null emphasis is noted on very weal; signals. Furthermore, nosignal-degradation takes place, as is the case for the more conventionalsquare-law sharpening method, in which interfering signals areemphasized.

While I have described the invention in detail for the preferred formshown, it will be understood that modifications may be made within thescope of the invention as defined in the claims which follow.

I claim:

I. Directionfinder means, comprising a radio receivcr including detectormeans and display means with means for presenting nulls of the envelopeoutput of said detector means, said receiver further including circuitmeans developing an A.-C. output and coupled to said detector means, theoutput of said circuit means being solely capacitatively connected toground, said detector means comprising a rectifier with germanium diodesin each of two grounded arms thereof, the diode polarity in one armbeing opposite to that in the other arm, and a load resistance in one ofsaid arms to the exclusion of the other of said arms.

2. Direction-finder means, comprising a radio receiver includingdetector means and display means with means for presenting nulls of theenvelope output of said detector means, said receiver further includingcircuit means developing an A.-C. output, a reference connectioncapacitatively isolated from said output, said detector means comprisingtwo parallel arms connected between said circuit output and saidreference connection, a half-wave rectifier in one of said arms, ahalfwave rectifier of opposite polarity in the other of said arms, and aload resistor in one of: said arms to the exclusion of the other of saidarms.

3. Direction-finder means according to claim 2, in which the rectifierin the arm including said resistor has a strong saturationcharacteristic in the vicinity of zero volts.

4. Direction-finder means, comprising a radio receiver includingdetector means and display means with means for presenting nulls of theenvelope output of said detector means, said receiver further includingcircuit means developing an A.-C. output, a reference connection, saiddetector means comprising two arms connected in parallel across saidoutput to said reference connection, a poled half-wave rectifier elementin one arm, two half-wave rectifier elements in the other arm andpolarized oppositely to that of said first-mentioned rectifier element,and a load resistor in that one arm which has the greater overallrectifier resistance.

5. Direction-finder means according to claim 4, in which the maximumresistance of the rectifier means in the arm containing the resistor isof the order of mag nitude of said resistor.

6. Directiomfinder means, comprising a radio re ceiver includingdetector means and display means with means for presenting nulls of theenvelope output of said detector means, said detector means comprising ahalf-wave rectifier element in series with a load resistance, saidrectifier element having a resistance characteristic increasing in thevicinity of zero input volts, whereby voltage output across said loadresistance is non-linearly de-emphasized in the vicinity of zero inputvolts.

7. Direction-finder means according to claim 2, in which said circuitmeans includes a tank circuit capacitatively isolated from saidreference connection, said parallel arms each being connected at one endto said tank circuit and at the other to said reference connection.

8. Direction-finder means, comprising a radio receiver including anintermediate-frequency circuit, a tank circuit connected to the outputof said intermediate-frequency circuit, and detector means connected tosaid tank circuit, said detector means comprising first and secondnon-linear arms in parallel, said first non linear arm being poled toclip positive voltages in tank circuit output, and said secondnon-linear arm being poled oppositely to said first arm, said second armcomprising non-linear resistance means and linear resistance means inseries, the maximum forward resistance of said non-linear resistancemeans being of the order of magnitude of said linear resistance means,and filter means at the output of said detector means.

References Cited in the file of this patent UNITED STATES PATENTS2,044,789 Hefele June 23, 1936 2,074,408 Lowell Mar. 23, 1937 2,285,044Morris June 2, 1942 2,301,607 Bollinger et al. Nov. 10, 1942 2,485,731Gruen Oct. 25, 1949

